VPN is a common use of connections setup for remote access. However, VPN is known for notorious setup and licensing issues. This section guides you on how to harden Debian by hardening VPN server.
These are the identified threats related to Debian software.
VPN servers may be configured to use PPTP Protocol.
VPN servers can use IPSec or L2TP alone.
VPN server can alter root system.
VPN server can be configured to use outdated cipher.
VPN server can be configured not to use TLS encryption.
Here are the list if actions to counter the issues.
VPN server must use TLS at all cost.
Avoid Point-to-Point Tunneling Protocol (PPTP) at all cost. Use OpenSwan or OpenVPN instead.
Those 2 protocols goes in hands-in-hands.
Jail the VPN Server so that it does not have access to actual root system.
Do not roll your own crypto. Use known strong cipher like ChaCha20-Poly1305 or AES-GCM.
That's all for hardening Debian by hardening VPN.